For a method of obtaining color hard copies, the color recording techniques of the ink-jet recording system, electrophotographic recording system, the thermal transfer recording system, and so forth have been studied.
Among the techniques, the thermal transfer recording system, in particular, has the advantages that the operations and maintenance can readily be performed, the apparatuses can be miniaturized, and the production costs can also be saved. In addition to the above, the inexpensive running costs can further be enjoyed as the other advantage thereof.
The thermal transfer recording systems of this type may be classified into the following two kinds; namely, one system, that is so-called a fusion transfer system, in which a transfer sheet comprising a support bearing thereon a fusible ink layer, (the sheet is hereinafter sometimes referred to as a thermal transfer material), is heated by means of a thermal head and the ink is then fusibly transferred over to a sheet subject to the ink transfer, (the sheet is hereinafter sometimes referred to as a transfer image recipient material); and the other system, that is so-called a thermal diffusion transfer system, in which a transfer sheet comprising a support bearing thereon an ink layer containing a thermally diffusible dye, (i.e., a sublimable dye), such transfer sheet is heated by means of a thermal head and the thermally diffusible dye is then transferred over to the transfer image recipient sheet. The latter, the thermal diffusion transfer system, is more advantageous to a full-color recording operation, because an image gradation can be controlled by varying an amount of a dye to be transferred so as to meet the thermal energy variations of a thermal head used.
In the thermal transfer recording operated in the thermal diffusion transfer system, the dyes applied to a thermal transfer materials are an essential element, because they are a great influence on a transfer recording speed, an image quality, and an image storage stability.
Therefore, the dyes applicable to the above-described thermal diffusion transfer system are required to have the following characteristics:
(1) The dyes are to readily be thermally diffused (i.e., they are to readily be sublimated) under the thermal recording conditions (such as the temperature of a head and the time of heating the head);
(2) The dyes are to have a hue desirable for a color reproduction;
(3) The dyes are not to be thermally decomposed at any heating temperature in recording operations;
(4) The dyes are to be excellent in resistance against light, heat, humidity, and chemicals;
(5) The dyes are to have a substantially greater molar absorptivity coefficient;
(6) The dyes are to readily be added to a thermal transfer material; and
(7) The dyes are to readily be synthesized.
In this invention, the term, `a thermal diffusion`, hereinafter means that an independent dye is substantially diffused and/or transferred when a thermal transfer material is heated and the dye is in the form of gas, liquid or solid depending on the heat energy applied; and the term is substantially synonymous with the term, `a sublimation transfer`, that is named in the art.
As for the cyan dyes for thermal transfer materials, naphthoquinone dyes, anthraquinone dyes and azomethine dyes have so far disclosed in, for example, Japanese Patent Publication Open to Pubic Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 59--78896/1984, 59-227948/1984, 60-24996/1985, 60-53563/1985, 60-130735/1985, 60-131292/1985, 60-239289/1985, 61-19396/1986, 61-22993/1986, 61-31292/1986, 61-31467/1986, 61-35994/1986, 61-49893/1986, 61-148269/1986, 62-191191/1987, 63-91288/1988, 63-91287/1988 and 63-290793/1988. However, there have not been any dyes found to satisfy all the above-given characteristic requirements. It have, therefore, been demanded to develop a cyan dye having each of the improved thermal diffusibility, hue, and the resistance against heat and light, and a thermal transfer material applied with the improved dye.
From the viewpoints mentioned above, the present inventors have devoted themselves to studying the dyes for thermal transfer materials and they have discovered unexpectedly that the compounds having the following formula I can satisfy the foregoing characteristic requirements and the compounds have an excellent hue. Based on the discovery, the invention could finally be achieved.